I recently read "Mastering Joomla! 1.5 Extension and Framework Development" by James Kennard, and I must say I really enjoyed it. Every time I read a book about Joomla! framework and extension development, I get all sorts of new ideas. This time was no exception. The book provides a thorough description of the topics surrounding Joomla! 1.5 extension development, and it is a great reference book. I would not recommend reading the book cover-to-cover, but I think it is a great resource to have handy when attempting to develop extensions for Joomal! 1.5. The book was written for experience PHP developers who already have a working knowledge of Joomla! For those wanting to learn Joomla! extension devleopment, I recommend starting with Joe Leblanc's book, "Learning Joomla! 1.5 Extension Development", and having "Mastering Joomla! 1.5 Extension and Framework Development" as a reference for getting more in depth with the framework.